1. Field of the Invention
This invention relates to serial printers, and more particularly to a device in a serial printer for driving a carriage on which a printing head is mounted.
2. Description of the Related Art
In order to drive a carriage which supports a printing head, a stepping motor is typically used. The stepping motor drives the carriage by driving a belt coupled to the carriage. However, even though a stepping motor can accurately position the carriage, a stepping motor is relatively expensive to manufacture.
On the other hand, a DC motor is a less expensive alternative means with which to drive the belt. In addition, a DC motor is usually contained in a printer of a large scale computer that prints a large amount of data in a short period of time. However, unlike a stepping motor, a DC motor has a rotary shaft which rotates at high speeds and provides a lot of torque. Therefore, since the high speed rotary shaft of the DC motor drives the belt, another device is needed to determine the distance that the belt has moved. An encoder is typically used to measure the distance moved by the belt and outputs a signal which represents the measured distance. Subsequently, the output signal and a separate clock pulse signal are utilized to synchronize the printing of the printing head on the carriage. The encoder may be a rotary encoder or a linear encoder, but, in order to print high quality characters at high speeds, a linear encoder is used in most instances.
In order to supply the output signal of the linear encoder from the carriage to the printer body, a flexible pattern circuit (FPC) is employed. An FPC is basically a flat cable constructed of a flexible substrate which contains conductive patterns on its surface. In addition to transmitting the output signal of the linear encoder, the FPC also supplies current from the printer body to the printing head in order to drive the printing head.
Since detecting the position of the carriage on the carriage drive shaft is essential to synchronize the operation of the printing head, a circuit which instantaneously calculates the position and the direction of the movement of the carriage from the output signal of the linear encoder is provided on the carriage. However, the carriage of a printer thus constructed is bulky and accordingly the carriage-driving device is bulky.
FIG. 7 is an illustration of the waveforms of the current supplied to the printing head and of the output signals transmitted by the encoder. As shown by the waveforms V.sub.A and V.sub.B, the encoder signals have a significant amount of noise at the rise and fall of the current I.sub.P that is supplied to the printer head.
FIG. 8 shows a portion of a conventional shielded FPC which is used to reduce the noise of the encoder signals V.sub.A and V.sub.B. The FPC is made of three separate layers. The top layer is a layer of conductive strips 21 which supply the current I.sub.P to the printing head. The bottom layer is a layer of conductive strips 22 which supply current to the encoder, transmit the output signal of the encoder to the printing body, and contain a ground.
The middle layer comprises an electrostatic shielding member 23. The electrostatic shielding member 23 prevents the current I.sub.P from creating noise in the output signals V.sub.A and V.sub.B of the encoder. However, the electrostatic shielding member 23 is expensive, and there has been a demand for an FPC which has a different structure and is less expensive than a convention FPC.
In view of the foregoing, an object of the invention is to provide a carriage-driving device for a serial printer which the connects the printer body to the carriage through a less expensive FPC.